This invention relates to an improved galvanometer unit. More particularly, it relates to a galvanometer unit incorporating a limited-rotation motor having an improved bearing life and improved position control for high-speed actuation.
A galvanometer unit to which the invention relates includes a limited-rotation electro magnetic motor having a permanent-magnet armature that interacts with the fields generated by currents through field windings. Motors of this type are often used in scanners, in which a light-directing component, usually a mirror, is attached to the motor shaft and reciprocal rotation of the motor causes a light beam directed at the mirror to sweep back and forth over a target surface.
Since the motor undergoes limited rotation, the rotor, which comprises the armature and associated shafts, may be mounted on a flexural pivot that acts as a torsional spring for motor rotation. However, the motor to which the present invention relates incorporates bearings to support the armature and the limitation on rotation is provided by the servo system that controls the angular position of the mirror. Galvanometer motors of this type have in the past suffered from bearing wear, which degrades the accuracy of light beam direction, ultimately reaching an unacceptable level and requiring replacement of the scanner.
Another problem encountered with prior galvanometer motors is the torsional resonance of the rotating system, i.e. the rotor, the load, e.g. mirror, and any other rotating components. A position sensor is connected to the shaft to provide position feedback in the servo loop and the output of the sensor includes components resulting from resonant twisting of the shaft. There are several resonance modes and the pass band of the servo system must be well below the lowest resonance frequency to avoid unwanted feedback causing instability of the servo system. Other problems to which the invention is directed are the desirability of stability and high sensitivity of the position servo. A further problem is the need for uniformity of temperature in the rotating system and efficient removal of heat from the motor.
A galvanometer unit incorporating the invention supports the armature on ball or roller bearings. A servo controller that rotates the scanner to commanded angular positions is programmed to cause the rotor to undergo one or more complete revolutions from time to time. This changes the angular relationships between the bearing balls or rollers and the inner and outer bearing races. Bearing wear is thus shifted to different portions of the races and wear is distributed around the races instead of being concentrated in a single angular span. This materially increases bearing life.
Preferably, also, the position sensor in the servo system is located at a null point of the fundamental resonance mode of the rotating system. Thus there is negligible feedback in the servo system from this resonance. This permits operation of the scanner at significantly higher speeds.
More specifically, the rotating system exhibits a fundamental torsional resonance mode in which the instantaneous angular velocities of the motor armature and the mirror are in opposite directions. The frequency of this mode, as well as the frequencies of higher order modes, is a function primarily of the rotational inertias and torsional stiffnesses in the rotating system. The fundamental mode has a single null at an axial position on the shaft determined by the physical parameters of the rotating components. The output of a sensor located at the null position contains a negligible frequency component corresponding to the fundamental resonance mode. Therefore, the pass band of the servo system, one of whose inputs is the angular position indicated by the sensor, can be increased to a frequency closer to the fundamental resonance than is practical in prior systems.
A further improvement is provided by the use of a capacitive position sensor that is thinner than prior sensors. This reduces the length of the shaft linking the scanning mirror to the motor, which results with a corresponding increase in shaft stiffness. This in turn increases the various resonances, including the fundamental resonance frequency, again permitting an increase in the pass band of the servo system.
A novel rotor structure and method of fabricating it contribute both to torsional stiffness and high electrical and thermal conductivity between the armature and the shafts in the rotating system. This facilitates grounding of the rotor to prevent the buildup of static charges and it also provides for temperature uniformity so as to minimize differential thermal expansion.